A method and a device for ascertaining the offset value of a longitudinal acceleration sensor are described in German Published Patent Application No. 10 102 54 296 A1. Here, a first inclination variable representing the roadway inclination is ascertained from the output signal of the longitudinal acceleration sensor during a vehicle standstill with an applied brake. After the brake is released following the vehicle standstill, a second inclination variable representing the roadway inclination is ascertained from the output signal of the sensor. The offset of the longitudinal acceleration sensor is ascertained as a function of the two inclination variables.
Frequently, very high demands are placed on the temperature dependency of the offset value of the longitudinal acceleration sensor used for ascertaining the roadway inclination for standstill functions acting on the longitudinal dynamics of a vehicle dynamics control system such as, for example, a hill hold control function. This means that the output signal of the sensor may only have a very weak temperature dependency. In FIG. 1, the offset value of a longitudinal acceleration sensor is shown, for example, as a function of the temperature. In the abscissa direction, the temperature of the sensor increases from left to right.
“LT” means low temperature in the range of −40° C., “NT” means normal temperature in the range of 20° C. and “HT” means a high temperature in the range of 85° C. In the ordinate direction, for example, the offset value is plotted. Letters a, b, c and d denote four different sensors.
As the temperature increases, the offset value in the example of FIG. 1 increases.
If a vehicle is driven for a long time at a high temperature, the offset value of the longitudinal acceleration sensor will increase in the example according to FIG. 1. This value is learned by the long-term offset compensation algorithms and is stored in the control unit when the ignition is off. When the ignition is on, the stored value is reused. However, during the time in which the vehicle was parked and switched off, the temperature is certain to have changed, and thus the prevailing offset. Since this changed offset for the vehicle or the system occurs abruptly when the ignition is on, a short-term offset is assumed, which is not immediately adjustable. This problem is addressed by setting very stringent short-term offsets, i.e., temperature characteristics, as requirements for the sensors. In the example according to FIG. 1, this might mean that only sensor a is considered; sensors b, c and d are, however, discarded because of too severe temperature dependency of the offset value.